Mechanical parts, such as automobile transmissions and vehicle bearings, receive a significant number of cycles of stress during service. Thus it is necessary to understand the number of such cycles of stress the metallic material, used for the mechanical part, will endure before service, and the metallic material should be designed based on the size, shape, service life, etc. of the mechanical parts. For metallic materials used in such mechanical parts, the fatigue limit is currently determined by considering that the material will last permanently without fatigue service if it holds up to 107 incidents of stress.
In recent years, however, a newly found phenomenon exists where some materials, running out the fatigue test up to 107 times, fail when they have received a stress which is lower than the fatigue limit of the conventional definition of more than 107 times. The fatigue strength of a metal depends on defects present in it as well as on its intrinsic strength. Defects serve as points where stresses concentrate, providing starting points for fatigue failure. Non-metallic inclusions (hereinafter, called “inclusions”) in metallic material are a type of such defects. Thus, in the conventional fatigue strength design, the stress concentration, caused by inclusions serving as fatigue failure starting points, is considered with reference to √(area)—which is the size of an inclusion expressed by the square root of its area.
Meanwhile, inclusions have the effect of trapping hydrogen in addition to stress concentration. Hydrogen, in a metal, is known to affect the microscopic failure mechanisms of the metal. This is particularly significant for high tensile steel. The fatigue area around an inclusion affected by hydrogen (i.e., the trapped-hydrogen-affected area) looks black because of its roughness when observed with a metallurgical microscope. This area is called an ODA (optically dark area). Some fatigue test results indicate that the trapped hydrogen lowers the fatigue strength of the area around an inclusion. In terms of strength, trapped hydrogen can therefore be considered to have the equivalent effect of substantially enlarging the size of inclusion.
As a result of an intensive study of trapped hydrogen by observation with the metallurgical microscopy, the ODA size is found to grow as the fatigue life is prolonged from about 105 to 108 times or more. The conventional fatigue strength design is, however, based on the initial size √(area) of the inclusion. Thus, this conventional service life design is not the best model for determining the estimated service life of the mechanical part.
The present invention provides a long life fatigue strength design method for a metallic material, which can design a mechanical part best matching the estimated service life by taking into account the growth in the ODA size corresponding to the assumed service life of the mechanical part.